Breast Milk - Composition

Composition

If nutrient supply is found lacking, content is obtained from the mother's bodily stores. The exact composition of breast milk varies from day to day, depending on food consumption and environment, meaning that the ratio of water to fat fluctuates.

During the first few days after delivery, the mother produces colostrum. This is a thin yellowish fluid that is the same fluid that sometimes leaks from the breasts during pregnancy. It is rich in protein and antibodies that provide passive immunity to the baby (the baby's immune system is not fully developed at birth). Colostrum also helps the newborn's digestive system to grow and function properly.

Colostrum will gradually change to become mature milk. In the first 3–4 days it will appear thin and watery and will taste very sweet; later, the milk will be thicker and creamier. Human milk quenches the baby's thirst and hunger and provides the proteins, sugar, minerals, and antibodies that the baby needs.

In the 1980s and 1990s, lactation professionals (De Cleats) used to make a differentiation between foremilk and hindmilk. But this differentiation causes confusion as there are not two types of milk. Instead, as a baby breastfeeds, the fat content very gradually increases, with the milk becoming fattier and fattier over time.

The level of Immunoglobulin A (IgA) in breast milk remains high from day 10 until at least 7.5 months post-partum.

Human milk contains 0.8% to 0.9% protein, 4.5% fat, 7.1% carbohydrates, and 0.2% ash (minerals). Carbohydrates are mainly lactose; several lactose-based oligosaccharides have been identified as minor components. The fat fraction contains specific triglycerides of palmitic and oleic acid (O-P-O triglycerides), and also quite a large quantity of lipids with trans bonds (see: trans fat) that are considered to have a health benefit. They are vaccenic acid, and Conjugated linoleic acid (CLA) accounting for up to 6% of the human milk fat.

The principal proteins are casein (homologous to bovine beta-casein), alpha-lactalbumin, lactoferrin (apo-lactoferrin), IgA, lysozyme, and serum albumin. In an acidic environment such as the stomach, alpha-lactalbumin unfolds into a different form and binds oleic acid to form a complex called HAMLET that kills tumor cells. This is thought to contribute to the protection of breastfed babies against cancer.

Non-protein nitrogen-containing compounds, making up 25% of the milk's nitrogen, include urea, uric acid, creatine, creatinine, amino acids, and nucleotides. Breast milk has circadian variations; some of the nucleotides are more commonly produced during the night, others during the day.

Mother's milk has been shown to supply a type of endocannabinoid (the natural neurotransmitters that marijuana simulates), 2-Arachidonoyl glycerol.

At one time it was thought that breast milk was sterile, however it is now known that it is more similar to cultured yogurt, containing as many as 600 different species of beneficial bacteria. It is also now known that breast milk contains a unique type of sugars, oligosaccharides, which are long chains of complex sugars. So far scientists have identified 140 of them and estimate there are about 200. These types of sugars are found nowhere else in nature, and not every mother produces the same ones, since they vary by blood type. However, the oligosaccharides are not digestible by infants and are instead meant to feed the beneficial bacteria that live in the intestine and help to fight infections. Also found in breast milk are endo-cannabinoids, which may act as an appetite stimulant, but they also regulate appetite so infants don't eat too much. That may be why formula-fed babies have a higher caloric intake than breastfed babies.

The breast milk of diabetic mothers has been shown to have a different composition from that of non-diabetic mothers. It may contain elevated levels of glucose and insulin and decreased polyunsaturated fatty acids. A dose-dependent effect of diabetic breast milk on increasing language delays in infants has also been noted, although doctors recommend that diabetic mothers breastfeed despite this potential risk.

Though it now is almost universally prescribed, in some countries in the 1950s the practice of breastfeeding went through a period where it was out of vogue and the use of infant formula was considered superior to breast milk. However, it is now universally recognized that there is no commercial formula that can equal breast milk. In addition to the appropriate amounts of carbohydrate, protein, and fat, breast milk provides vitamins, minerals, digestive enzymes, and hormones. Breast milk also contains antibodies and lymphocytes from the mother that help the baby resist infections. The immune function of breast milk is individualized, as the mother, through her touching and taking care of the baby, comes into contact with pathogens that colonize the baby, and, as a consequence, her body makes the appropriate antibodies and immune cells. Breast milk contains less iron than formula, as iron is an essential nutrient for the survival of pathogens inside a host. However, the iron supplied in breast milk is more available to the infant than that supplied in formula feedings or supplements. At around four to six months of age the internal iron supplies of the infant, held in the hepatic cells of the liver, is exhausted, hence this is the time that complementary feeding is introduced.

Women breastfeeding should consult with their physician regarding substances that can be unwittingly passed to the infant via breast milk, such as alcohol, viruses (HIV or HTLV-1) or medications.

Most women that do not breastfeed use infant formula, but breast milk donated by volunteers to human milk banks can be obtained by prescription in some countries.

Mother's breast milk provides a higher proportion of cholesterol than almost any other food. It also contains over 50% of its calories as fat, much of it saturated fat. Both cholesterol and saturated fat are essential for growth in babies and children, especially the development of the brain. Most commercial formulas are low in saturated fats and soy formulas care completely devoid of cholesterol. A recent study linked lowfat diets with failure to thrive in children.